Conventionally, an AlGaN-based nitride semiconductor includes a light-emitting element or a light-receiving element having a multi-layer structure formed on a base of a GaN layer or an AlGaN layer having a relatively low AlN mole fraction (also called an AlN mixed crystal ratio or Al composition ratio) (refer to Non-patent Document 1 and Non-patent Document 2, for example). FIG. 16 shows a conventional crystal layer structure of an AlGaN-based light-emitting diode. The light-emitting diode shown in FIG. 16 has a laminated structure in which a base layer 102 including an AlN layer is formed on a sapphire substrate 101, and on the base layer 102, an n-type clad layer 103 composed of n-type AlGaN, an AlGaN/GaN multiple quantum well active layer 104, an electron block layer 105 composed of p-type AlGaN, a p-type clad layer 106 composed of p-type AlGaN, and a contact layer 107 composed of p-type GaN are sequentially laminated. The multiple quantum well active layer 104 has a structure having multiple-layer structures each having a GaN well layer sandwiched between AlGaN barrier layers. After crystal growth, the multiple quantum well active layer 104, the electron block layer 105, the p-type clad layer 106, and the p-type contact layer 107 are partially etched away until a part of a surface of the n-type clad layer 103 is exposed. After that, a p electrode 108 composed of Ni/Au is formed on a surface of the p-type contact layer 107, and an n electrode 109 composed of Ti/Al/Ti/Au is formed on the exposed surface of the n-type clad layer 103. By changing an AlN mole fraction and a thickness of an AlGaN well layer in place of the GaN well layer, a light emission wavelength is reduced, or by adding In, the light emission wavelength is increased, so that the light-emitting diode having an ultraviolet light region from a wavelength of 200 nm to 400 nm can be manufactured.